HAL Id: hal-03198072
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Designing a new posthole seismological station on Antarctica inlandsis (Concordia station)
Maxime Bes de Berc, Alessia Maggi, Jean-Jacques Leveque, Jean-Yves Thoré
To cite this version:
Maxime Bes de Berc, Alessia Maggi, Jean-Jacques Leveque, Jean-Yves Thoré. Designing a new post-hole seismological station on Antarctica inlandsis (Concordia station). AGU Fall Meeting, Dec 2015, San Francisco, United States. �hal-03198072�
ABSTRACT
Concordia (75°S 123°E) is a scientific base operated by French and Italian polar institutes IPEV (Institut PaulEmile Victor) and PNRA (Programma Nazionale di Ricerche in Antartide, and is located on the inlandsis of the East Antarctica plateau.
It hosts a seismological station CCD which has provided observatory quality data since the year 2000, has been integrated into the Geoscope network since 2008 and whose data are now available in realtime from IRIS. The current seismic vault is located 800m from Concordia base, as far away as is deemed logistically possible by IPEV, at a depth of 12m. The vault is thermally very stable, but given the close distance to the base, suffers from increased diurnal noise (up to 40 dB) at frequencies above 1Hz, especially in the summer season. Anthropic noise is trapped in the firn (snow) layer, which forms an 100110m thick waveguide, and is picked up very easily in the seismic vault. The vault is made from steel containers buried vertically in the snow. The hydrostatic pressure of the snow is deforming them: we see container cracking events on the seismograms, and also visual evidence of container deformation inside the vault. In the near future, this deformation will create a security problem.
We have decided to progressively abandon our current vault, and construct a new posthole seismological installation nearby. We plan to drill to 130m depth, which would place us below the firn layer waveguide and also below the ice pinchout depth. To be able to run the station for several years and change or service the instrumentation if required, we need to keep the hole open, to avoid any hydrostatic movement, and to maintain good coupling between the sensor and the surrounding hard ice. To achieve these goals, we shall install a casing in the fin layer and then drill a few meters more without casing in hard ice. After installing the instrument, we shall then fill the whole hole with a drilling fluid whose density is similar to that of ice.
The drilling will be carried out by the French drilling facility C2FN (Centre de Carottage et de Forage National) of the CNRS (Centre National de la Recherche Scientifique), and the is planned for the 20182019 summer season.
Designing a new posthole seismological station on Antarctica inlandsis (Concordia station)
Bès de Berc M.
(1)(mbesdeberc@unistra.fr), Maggi A.
(2), Lévêque JJ.
(1), Thore JY.
(3)(1)
IPGS, Institut de Physique du Globe de Strasbourg, Strasbourg, France,
(2)UNISTRA, Université de Strasbourg, Strasbourg, France,
(3)EOST, Ecole et Observatoire des Sciences de la Terre, Strasbourg, France
PLANNED SENSOR INSTALLATION
Figure 1:
The firn (clear blue) is about 110m deep.
It acts as a waveguide for anthropic
noise (Anderson et al., 2003). Our goal
is therefore an installation within the
hard ice below (dark blue).
Figure 2:
The first step will be to drill at a depth a
few meters below the interface between
firn and ice. We therefore chose 130m.
(D=250 mm)
Figure 3:
The casing is installed, avoiding the hole
closing. It's made in HDPE tubes
(D=225 mm, d=204,5mm)
Figure 4:
An other drill is performed only one
meter below the casing, directly in the
hard ice.
The
completion
of
drilling
and
installation of casing will be performed
by the french drilling facility C2FN.
Figure 5:
The sensor is classically installed into
the hard ice, without any contact with
the casing. The part of the hole without
casing will naturally close but in a much
longer time than the firn, as it takes
several
years
into
the
hard
ice.
Eventually, we plan to fill with silicone
based oil as drilling fluid.
BIBLIOGRAPHY
Anderson, K., Aster, R., Butler, R., Hutt, B., Storm, T., Anderson, D., Vineyard, J., Albert, D., (2003). A new quiet GSN site at the South Pole: Comparison of seismic data between SPA and QSPA, American Geophysical Union, Fall Meeting 2003, abstract #C41C0982.
Anderson K. (2003). Seismic Noise Trapping in Firn, GEOP 523/Theoretical Seismology Class Presentation Nanometrics (2015). Trillium Posthole User Guide, Technical Manual
PERSPECTIVES
A new shelter on piloti (to limit snow accumulation) will be installed during 20172018 summer season, by IPEV technical staff.
The drilling and seismometer installation will be realized during 20182019 summer season, by C2FN staff. For the next following seasons, we plan to buy suitable instrumentation and perform several calibration tests on surface, in order to check how it supports extreme conditions and its instrumental noise at f>10Hz. We also will buy equipments needed to perform the installation (casing, tripod, etc...) as logistics in Antarctica must be planned a long time ahead.
CURRENT SITUATION
NOISE ANALYSIS AND EXPECTED GAIN
Upper left:
Location of Concordia on East
Antarctica Inlandsis (75°S, 123°E, 3200m) Upper right:
Overview of Concordia station from the instrumentation shelter, 800m far away.
Center:
Current installation of STS2
seismometers in 12m deep niche. The temperature is very stable at 54°C.
Bottom left:
Current instrumentation shelter. This shelter is nowadays completetely snowy.
The cave is accessible through a hatch into the shelter and a 45m long gallery 1m below the surface. Then, a ladder allows going down into the cave.
